This invention relates to organic phosphors comprising molecules of at least one phosphorescence activator held within a matrix of a condensation resin that does not itself absorb ultra-violet or visible radiation at the excitation and emission frequencies of the phosphorescence activator.
Phosphors of such type are known for example from British Patent Specifications Nos.870,504, 1,494,102 and 1,494,103. Such phosphors may contain as a matrix a cross-linked condensation resin, the resin being formed by condensation in the presence of phorphorescence activators so that the molecules of the latter are trapped and isolated from one another in the matrix. The resin is preferably a condensation product of formaldehyde, generally with an amine group-containing compound, especially urea or melamine. The phosphor is a solid generally insoluble in water and organic solvents and for use as a phosphorescent printing ink may be ground finely and dispersed in a suitable solvent, for example toluene.
The phosphorescence activators which have hitherto been proposed fall within a variety of classes of organic compounds. Examples of such compounds include carbazole sulphonic acid, naphthalene disulphonic acid, diphenyl guanadine and 2-aminobenzoflavone. However the mean intensity of the phosphorescent emission obtained with such compounds is often mediocre for practical use in for example security marking of documents and products.